Atari were cool to anyone growing up in the 70s and 80s both with their home computers, and arcade machine coin-ops, "Asteroids", "Missile Command", "Tempest", "Paper Boy", "Gauntlet" etc etc were all huge successes. It is called "Retro" clothing, don't see why it would get ice cold, there is plenty of retro clothing out there at the moment....this would appeal to anyone 30+ that has a geek interest, I would wear it personally.

I had a 2nd hand 2600 and later on (because I couldnt get hold of an Amiga A500) I bought a 512 ST ... with single sided external floppy drive. Just because something was made before you were (probably born) doesn't mean it isn't cool. It certainly is cool for those of old enough to remember that games lived or died on their playability rather than the pretty graphics overload.

I would guess it's because the Atari that exists today is almost entirely unrelated to the company that produced consoles back a few decades ago?
Atari is now just a name to splash on (predominantly) very poor games.

That would be my best guess. Voted hot though, I think 15 quids a fair price to pay for a hoody with a band name, etc on.